The present invention relates to a process for preparing polymers by means of anionic polymerization. More particularly the present invention relates to a new method for neutralizing the basic species remaining after the termination of such anionic polymerization reactions.
It is previously known in the art to prepare polymers of monovinylidene aromatic monomers and alkadiene monomers via anionic polymerization. Highly useful polymers are prepared in the form of block copolymers containing one or more blocks of a monovinylidene aromatic polymer and one or more blocks of an alkadiene polymer. Suitably such polymers are prepared in an organic solvent.
The initiators use in the anionic polymerization of monovinylidene aromatic monomers and alkadiene monomers are typically very strong bases. Examples are the alkali metal organyl compounds, particularly sodium and lithium alkyls. After polymerization is complete, the terminal monomer group of the living polymer anion may be terminated, for example, by reaction with a suitable proton donating agent such as an organic alcohol, organic acid or an epoxy containing compound. This results in the formation of an alkali metal containing remnant, either an alkali metal alkoxide or alkali metal carboxylic acid salt. In addition the living polymer may be coupled. During the coupling process termination may or may not also occur depending on the type of coupling agent employed. The use of polyhalide coupling compounds, such as silicon tetrachloride, carbon tetrachloride, ethylene dibromide, etc., results in both coupling and termination of the living polymer anion. If a polyvinyl compound, such as divinyl benzene is employed to couple the polymer anion, the resulting product remains a living anion which must still be terminated with a proton donating species. If a polyepoxide coupling compound is employed, such as epoxidized soybean oil or epoxidized linseed oil, the resulting polymer contains a remnant lithium alkoxide functionality and the polymerization is simultaneously terminated. Processes for coupling living polymers by the use of epoxides have been previously disclosed in U.S. Pat. No. 3,639,517, the teachings of which are incorporated herein by reference thereto.
Whenever any of the foregoing techniques of polymer termination or coupling are employed which result in the formation of an alkali metal alkoxide remnant it has now been discovered that a deleterious effect may result under certain circumstances due to an undesired reaction between such remnants and additives incorporated into the mixture. In particular in the industrial preparation of block copolymers it is often desirable to include an antioxidant in the polymeric syrup to prevent oxidative and mechanical degradation of the polymer during devolatilization and finishing. However, it has now been discovered that many of the suitable antioxidants employed in such formulations can react with the alkali metal remnant resulting from the forgoing termination or coupling reaction thereby forming undesirable products or at least inhibiting the ability of the antioxidant to prevent oxidative degradation of the resulting polymer.
Consequently, certain polymer products which contain alkali metal remnants and an antioxidant have been found to be lacking in both color retention and in aging stability. In particular it has been found that after further mechanical and thermal treatment the melt viscosity of certain polymeric products may change. In addition such polymeric products tend to have increased yellowing and physical properties, such as ultimate tensile strength, may be adversely affected.
In U.S. Pat. No. 4,415,695 it is proposed to employ boric acid as a neutralizing agent in an anionic polymerization. Disadvantageously when boric acid is employed as a neutralizing agent in the preparation of a block copolymer of a monovinylidene aromatic monomer and an alkadiene, certain product formulations may still possess an undesirable increase in melt viscosity upon long term thermal aging.
It would be desirable if there were provided an improved process for preparing polymers by means of anionic polymerization techniques employing a method of neutralizing the remnants formed by reaction of a terminating agent and the alkali metal polymeric anion that does not adversely affect the polymer properties.